About one third of the land on the earth belongs to an arid region, and further expansion of the arid region due to further global warming is expected. Also to address serious food shortages due to population growth, the development of techniques to improve, maintain, and increase plant yields are urgently needed in regions that are arid, accumulate salt, or have a high or low temperature for plants, i.e., regions where in a conventional manner plants are unlikely to grow or growth is hindered and yields are reduced.
When growing plants in a natural or artificial environment, they are subjected to a variety of stresses such as temperature (high temperature, low temperature, freezing), the severity of wind, light intensity (strong light, weak light), dryness, the toxicity of inorganic substances (e.g., salts, heavy metals, and aluminium), lack of oxygen, mechanics (hard soil), and pests. However, plants, unlike animals, cannot migrate to protect themselves from a variety of stresses. It is known that plants therefore produce, when they are subjected to a stress, a variety of substances in their bodies in order to develop stress tolerances, for example, compatible solutes such as proline, glycine betaine, and sugars (Non-patent Document 1). Moreover, it is known that when plants are subjected to such stresses, they produce an aging hormone such as abscisic acid to slow or terminate growth, and as a result yields are reduced.
Methods for enhancing such stress tolerances of plants include a method that involves selection and breeding as well as gene modification (See Patent Document 1) and application of plant energizers such as sugars, organic acids, and amino acids (See Patent Document 2). Further, it is also known that catechins are effective plant growth accelerators (See Patent Documents 3 and 4, for example).